Intracellular receptors are a superfamily of related proteins that mediate the nuclear effects of a variety of hormones and effector molecules, including steroid hormones, thyroid hormones and vitamins A and D. Members of this family of intracellular receptors are prototypical ligand activated transcription factors. These receptors contain two primary functional domains: a DNA binding domain (DBD) that contains about sixty-six amino acids and a ligand-binding domain (LBD) located in the carboxyl-terminal half of the receptor that has about 300 amino acids. The receptors are inactive in the absence of hormone (ligand) by virtue of association with inactivating factors, such as heat shock proteins. Upon ligand binding, the receptors dissociate from the inactivating complex and dimerize, which renders them able to bind to DNA and modulate transcription.
For example, for the steroid receptors, binding of a steroid hormone to its receptor results in receptor protein homodimerization and subsequent binding to the “steroid response element” (SRE) DNA sequence in nuclear DNA. Conformational changes in the receptor associated with ligand binding results in the recruitment of other transcriptional regulatory proteins, called co-activators, that regulate the transcription from promoters adjacent to the SRE binding sites.
Modified steroid hormone receptors have been developed for use for regulated expression of transgenes (see, e.g., U.S. Pat. No. 5,874,534 and published International PCT application No. WO 98/18925, which is based on U.S. provisional application Ser. No. 60/029,964) by modifying the ligand specificity of the LBD. In addition, the DNA binding domain of the receptor has been replaced with a non-mammalian DNA binding domain selected from yeast GAL4 DBD, a viral DBD and an insect DBD to provide for regulated expression of a co-administered gene containing a region recognized by the non-mammalian DBD. These constructs, however, have several drawbacks. The non-mammalian DBD is potentially immunogenic and the array of sequences recognized by these DBD is limited, thereby severely restricting gene targets.
Therefore, there remains a need for more versatile gene regulators. It is an object herein to provide polypeptides that function as versatile regulators of gene expression.